The fire caused heavy damage to a three-story building that served as headquarters for a non-profit that, among other things, has provided legal support to thousands of hurricane and flood victims in Houston and beyond.

Video footage taken by witnesses and posted on Twitter showed the first floor of the building at Fannin and Bell streets in flames and thick black smoke rising from it. One witness also reported hearing some type of explosion before the fire broke out.

A firefighter was apparently injured and was taken to a hospital, firefighters told The Houston Chronicle. No updates were available Monday afternoon.

Firefighters battled the blaze from just past 9 a.m. to around 1 p.m.

Witness Christopher Hisle spotted the blaze out of his apartment window at 9:12 a.m. and shot dramatic photos of the blaze and of firefighters who he said arrived about 12 minutes later. A downtown station is only a few blocks away.

It was not immediately known if the blaze was connected to the Harvey-related flooding.

Normally on a Monday, the building would have been filled with clients and with 50 lawyers and staff, many of whom have helped Texans in the past with legal issues that include filing flood and hurricane insurance claims and getting out of leasese to damaged apartments after previous major storms, including Hurricane Ike and the more recent Memorial and Tax Day floods.

But the offices had been closed because of Hurricane Harvey.

Rich Tomlinson, litigation director for Lone Star Legal, said the blaze began on the backside of the first floor of the three-story building, but he had no idea of the cause or the extent of the damage. It's clear, however, that the office will not be able to immediately reopen to address the legal needs that are likely to arise in the immediate aftermath.

Even so, Tomlinson said he expects the legal staff to continue to find ways to help hurricane victims - even as the non-profit itself will need to figure out how to recover from a blaze that badly damaged a building that has served as their headquarters in downtown Houston for about two decades.

"I assume we have insurance and there's got to be some way to rebuild," he said. "I don't know how much damage has been done at this point."